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Not routinely dangerous in the sense that we risk our lives every time we venture outside, but insofar as individuals with grievances can exact a terrible penalty on strangers, especially in nations that place a high value on liberty. It is simply not possible to prevent every criminal, every terrorist, every nut from acting on his sociopathic instincts every time.

If we had forgotten that lesson over the past 11-1/2 years, we were given the chance to learn it again on Monday, and in an acutely painful way. The bombs that exploded in Boston on Patriots Day, at the finish line of the Boston Marathon, killed at least three people, including an 8-year-old boy. Many of the dozens who were wounded are said to have sustained gruesome, life-altering injuries. President Obama on Tuesday said the attack is being investigated as an act of terrorism.

It is critical that the country does what Obama vowed in the aftermath of the bombings: that it bring to bear the “full weight of justice” on those responsible. The deaths and suffering of innocents demand it. Equally important, it is hard enough to interdict every effort to commit mass murder; it will only encourage more such attacks if those bent on harm ever got the message that the nation’s response would be less than full bore.

It is also important to review the security procedures that Boston undertook leading up to the marathon. Such events are easy and obvious targets for terrorists, and since the terror attacks of 2001, cities have learned how to prepare for them. Were all appropriate steps taken this time? If not, Boston – and all cities – need to review their processes. If so, there may be new lessons to learn about weaknesses that can be exploited.

Or maybe not. There hasn’t been a successful terror attack on U.S. soil since 9/11, but that hasn’t been for lack of trying. Richard Reid, the would-be shoe bomber, was foiled on a trans-Atlantic flight only months after 9/11. Umar Farouk Abdulmutallab, the would-be underwear bomber, tried to bring down a plane near Detroit. In September 2009, police thwarted an effort to explode bombs in the New York City subway system. In 2010, police found a car bomb in the city’s Theater District. It didn’t explode.

Through some combination of good police work and good luck, the country remained unscarred between those attacks and Monday’s in Boston, if you don’t include mass shootings such as the one carried out by Maj. Nidal M. Hassan at Fort Hood, Texas, in 2009.

But, as has been observed many times and in various contexts, countries trying to prevent attacks have to get it right every time; terrorists have to get it right only once. Sometimes, they will.

It has become more difficult for terrorists to attack targets that have been hardened since 9/11, airplanes and government buildings among them. Large events like the Boston Marathon offer softer targets that are more difficult to secure. Even there, police have developed strategies to diminish the chance of attack, but the odds are against stopping every plot, every time.

This is a terrible moment for Boston and, indeed, for the country. For the victims, including the injured and those who lost loved ones, the trauma can’t be less than overwhelming. All will need support in the weeks ahead.

Boston will recover, as New York City did. Its citizens aren’t famous for giving any quarter to those intent on tyrannizing them. In that regard, we hope that planning for next year’s marathon proceeds as usual, with more thought given to security, but mainly focusing on the democratic right to gather for any peaceable reason.

Meanwhile, law enforcement from Boston to Washington needs to make this case a top priority and keep it there until Americans are able to look on the face of the kind of person who takes pleasure in the indiscriminate killing of strangers.